1. Field of the Invention
The present invention relates to a circuit for the compensation of the horizontal component of the earth's magnetic field for a color picture tube of a high-resolution monitor.
As is known, color picture tubes form the image on the screen by electronic excitation of the phosphoruses deposited on the internal face of the picture tube's glass.
Such phosphoruses are deposited in triplets in three different colours (red, green and blue).
They are excited separately by three different guns and, by suitably dosing such excitations, it is possible to reproduce all the colors of the spectrum.
To allow each electronic beam to strike the correct phosphorus the picture tube is provided with a mask with holes.
Each of the holes of the mask, positioned in front of each triplet of phosphoruses, performs the task of guiding the electronic beam onto the correct phosphorus.
Any excess electrons that do not pass through the holes are absorbed by the mask, which is made of metal material (normally low-carbon steel).
For the correct formation of the image on the screen, it is of course necessary that the gun of each color hit exactly and only the phosphoruses which it is meant to hit and this normally occurs when the beam's electrons are not deflected along their trajectory from the cathode to the phosphorus.
One of the possible causes of deflection is constituted by the presence of a magnetic field whatsoever, having a position and intensity such as to be capable of influencing the movement of the electrons. Such influence is governed by the Lorenz Law.
In particular the earth's magnetic field, present in every part of the globe and which cannot be eliminated, has an intensity such as to be capable of deflecting the trajectory of the electrons in a significant manner.
At each point of the earth such field has a vertical component, perpendicular to the earth's ground, highest at the two poles and nil at the Equator, and a horizontal component parallel to the earth's ground, nil at the two poles and highest at the Equator. The latter can in turn be split into two components perpendicular to one another.
If the center of the picture tube is taken as a reference, it shall therefore be subjected to three magnetic field components, a horizontal component Bx (lateral magnetic field) perpendicular to the axis of the picture tube, a horizontal component Bz (axial magnetic field) coincident with the axis of the picture tube and a vertical component By (vertical magnetic field).
The component Bx shall be highest when the picture tube shall be facing East or West and nil towards North or South.
The component Bz shall be highest when the picture tube shall be facing North or South and nil towards East or West.
The component By, nil only at the Equator, shall be directed downward in the Northern Hemisphere and upward in the Southern Hemisphere and its intensity, depending on the latitude of the picture tube's location, shall rise while moving from the Equator towards one of the poles.
The picture tube is assembled with its deflection yoke and calibrated under certain conditions of external magnetic field.
If, however, such conditions are altered, for example, if the place of use is at a latitude other than that of the manufacturing location, or if quite simply the picture tube is rotated on itself, the trajectory of the electrons shall undergo a deflection.
A first effect of such an alteration is indicated as an error of convergence. This is manifested when, due to an error in the trajectory before passing through the mask, the electrons hit the wrong holes and thus the wrong triplets.
The phosphorus that has been hit may be of the correct color, but its position may be such as not to contribute in forming the desired synthesis color.
If, for example, in a certain area green is not perfectly convergent, a point that should be white shall be displayed as two separate points, one violet, consisting of red are blue which are convergent, and the other green.
The distance between these two points gives a measure of this error.
A second effect of such an alteration is indicated as a landing error. This is manifested when the electrons pass through the mask at an angle other than the correct one and the beam passing through the mask is not centered with respect to the phosphoruses it is meant to hit.
If this non-centering of the beam with respect to the phosphorus is less than or equal to a certain set threshold, there is no visible effect on the image since the phosphorus is properly illuminated all the same.
Otherwise two cases can occur.
If the centering is such that the beam also hits the adjacent phosphorus, two phosphoruses of two different colors are triggered, giving rise to an error of purity.
On the other hand, if the non-centering is such that the beam, while not hitting adjacent phosphoruses, hits the correct phosphorus only partially, there is in the affected area a reduction in the quantity of light emitted and the uniformity of brilliancy of the image is thus jeopardized.
Thus, while it is substantially possible to ignore the effects of variations of latitude on the magnetic field's vertical component By, since such variation occurs only when the picture tube is installed and is evidently compensated directly by the installing technician, the same cannot be said for the horizontal component, which shall vary every time the user shall decide to change the picture tube's orientation.
2. Description of the Related Art
According to the known art, to limit the influence of the earth's magnetic field the picture tube is provided with an internal screen connected to the mask with holes, normally made of the same material.
Due to its low magnetic permeability, there is associated with the screen a demagnetization system which uses one or two coils located to the side of the picture tube, in which an alternating current is made to flow with an amplitude such as to cause the magnetic saturation of the screen's material in both directions.
Such magnetic field is then gradually reduced to zero.
In this way the material is made to operate in an area within the hysteresis cycle, with a much higher permeability and thus with a much more effective screening power.
It is obvious that such a screen works on the field's Bx and By components, while the only screening of the Bz component is that offered by the mask with holes, which is much less effective than the screen itself due to its limited thickness and to the holes drilled in it.
Moreover, the effectiveness of this internal screen, once demagnetization has been carried out, remains unchanged only in the absence of changes in the external magnetic field. If, after demagnetizing the screen, the picture tube is rotated, the screen shall no longer act as such and in some cases shall even make the effect of the altered external field conditions worse.
In such a case it is necessary to perform another demagnetization to adjust the internal screen to the new external magnetic conditions.
In television sets such a demagnetization system is activated automatically only when the set is turned on and no external reactivation is provided for since, once it is installed, the set is not normally moved again.
Only in the case of picture tubes for graphic monitors, especially in those larger than 16 inches, which are destined for a different use, in addition to automatic demagnetization at switch on there is also the possibility for the operator himself, by means of a special external knob, to demagnetize the set whenever he feels it is necessary.
Moreover, if the magnetic component Bz of the magnetic field in which the picture tube is to operate is different from that for which the picture tube has been calibrated at the plant, there shall be a drop in performance even after the set has been demagnetized.
The cause for this, as has been said earlier, is in the scant effectiveness of the mask in screening the component Bz of the external field.
It should be remembered in passing that picture tube manufacturers guarantee convergence and uniformity of brilliance only for a given value of magnetic field.